Field of the Invention
The present invention relates to an imaging device including a phototransistor which outputs photocurrent in light irradiation, a method of driving the imaging device, and a camera including the imaging device.
Description of the Related Art
CMOS (Complementary Metal-Oxide-Semiconductor) image sensors have been developed as highly sensitive imaging devices capable of imaging in the dark. A CMOS image sensor having a wide dynamic range capable of imaging in the dark and light at the same time has also been developed. A photodiode which outputs an electric charge to a photon on a one-to-one basis has been used for such CMOS image sensors as a photoelectric conversion element. CMOS image sensors convert signal charge generated through light receiving into voltage by source-follower output. Since the photodiodes have low sensitivity, signal charge is accumulated in, for example, a junction capacitance of the photodiode except in a readout time, in order to increase the signal charge generated by light energy. A method of increasing readout signal charge by signal charge accumulation has been adopted, so as to achieve high sensitivity. Regarding wide dynamic range, a technique which can correspond to high illumination imaging with an accumulation operation by the following method has been developed (refer to Patent Literature 1 (JP3984814B), for example).
(1) A method of photographing several images by a global shutter method which adjusts an accumulation time, and synthesizing the images.
(2) A method of having several capacitances in addition to floating diffusion.
(3) A method of using a photodiode having a capacitance larger than that of a conventional photodiode.
As one example of the above methods. Patent Literature 1 discloses a technique which connects a small capacity floating diffusion on the side of high sensitivity and low illumination and a large capacity floating diffusion on the side of low sensitivity and high illumination to a photodiode, selects the output on the side of low illumination and the output on the side of high illumination in accordance with an image-loading mode, and outputs each output. A wide dynamic range is thereby obtained by reducing 1/f noise.
A technique using a phototransistor having an operation of amplifying photocurrent as a photoelectric conversion element has also been developed (refer to Patent Literature 2 (JP H05-236196A), for example). Patent Literature 2 discloses a method of reading out signal charge after transferring the signal charge accumulated in the phototransistor to a capacitance and/or a base of a bipolar transistor provided separately from the phototransistor because all of the signal charge accumulated in the phototransistor cannot be discharged in the readout time. The accumulated charge remaining in the phototransistor in the transferring period of the signal charge from the phototransistor is reset.
However, with the conventional accumulation method, when light energy is strong and/or an accumulation time is long, the electric charge amount to be generated is increased, so that the electric charge amount exceeds the electric charge amount, which can be accumulated in the capacitance of the photoelectric conversion element, resulting in saturation. It becomes difficult to obtain a wide dynamic range. In order to solve this problem, it is necessary to increase the capacitance. It is also necessary to increase the area of the photoelectric conversion element, or to provide a capacitance which accumulates in another place for each pixel. For this reason, the size of an imaging device is increased. When a wide dynamic range is obtained by photographing and synthesizing a plurality of images with a change in irradiation time of light energy and/or accumulation time of signal charge, an operation of accumulating electric charge and a process speed at the time of synthesizing an image are required. For this reason, a circuit design is complicated in a data-processing portion, and an improvement in a device performance to be used is desired. Unfortunately, in the conventional technique, in order to obtain high sensitivity and a wide dynamic range, the circuit is complicated, and the size of the imaging device is increased.